I'd guess there are three types of murder cases.

1. Somebody got killed in their house or on the street. Cops don't know who was around and/or no one is talking. Case will not be solved. No amount of detective work will unravel the crime, best case is someone will offer information about it to get a plea deal in an unrelated case and it might lead to a conviction. Most the murders in my neighborhood work along these lines (gang related drive by).

2. The cops know something about who was around at the time of the crime or who someones enemies were. They start bringing people in, tell them they aren't suspects, do some questioning. Eventually a guilty party or someone with information tells a dumb enough lie for them to get an indictment.

3. A case like this one. Lots of real detective work to do on a high profile case that must be solved. I wonder how many of these cases are riddled with lots of police and prosecutor mistakes, I'd guess that most of them are. Cops don't expect that any case will turn into a real puzzle, they don't show due care because if they did every murder investigation would take an ungodly amount of man and brain power. I'm sure that as soon as they found the body the lead investigator was counting the hours until they got a confession out of the Dad. If a girl is dead and you think the boyfriend did it, the boyfriend did it, it's just how this kind of thing works.

I don't know that the author made any kind of case about why this investigation still matters, maybe he didn't get to write his own title.